Recently, in an antifouling paint and other areas, public attention was directed to the use of resinous filler particles which can be hydrolyzed in an ionic atmosphere, as in sea water of a weak alkaline condition, and gradually decomposed and dissolved out. These characteristics are specifically beneficial for prolonging the antifouling and polishing effects of an antifouling paint and attaining energy saving in sailing and the like.
Various resins have been proposed for this end as, for example, acrylic resins with halogenated aliphatic acid bondings or electron attractive group-containing acyl bondings (e.g. Japanese Patent Appln. Nos. 101,463/81 and 198,240/81); acrylic resins containing organic tin salts (e.g. Japanese Patent Appln. Kokai No. 98570/82); and polyester resins having metal ester bondings in the polymer main chain (e.g. Japanese Patent Appln. Nos. 165,921/81 and 165,922/81) and the like.
However, they were merely developed as resinous vehicles for antifouling paints, requiring film-forming properties and optimum film performance and therefore, there were in fact various limitations on the employable resins in respect to the molecular weight range, metal content and the like, in addition to the hydrolysis nature thereof.
In a coating composition area, attempts have also been made to add, to a film-forming resinous varnish, resin powders for improving the application characteristics, without unduly increasing viscosity, of the coating composition.
Therefore, even in an antifouling paint, hydrolysis type resin powders having no film-forming properties have been actually examined. For example, in Japanese Patent Publication No. 3830/86, there are disclosed film-forming polymer compositions comprising a polyacrylic acid salt having the constituting unit of the formula: ##STR2## in which M stands for Cu or Zn.
It is stated that said polyacrylic acid salts may be of a film-forming type or of a non-film-forming type and the molecular weight is in a range of 5000 to 1.times.10.sup.6. Therefore, it is clear that hydrolysis type crosslinked resins having no film-forming properties are likewise suggested in this publication. However, in preparing said resins, a specific method is used, wherein a carboxyl bearing acrylic resin is first neutralized with caustic soda and dissolved in an aqueous medium and thus obtained polymer solution is reacted with a metal salt, thereby forming a precipitate of an insoluble polyacrylic acid salt. In this type of reaction, a soluble resin is gradually changed to an insoluble type with the progress of the ion-exchange reaction, and the formed insoluble resins are precipitated as amorphous masses each varying in size and shape. Since a smaller precipitate has a larger surface area and more rapidly hydrolyzed with sea water than a larger one, when the aforesaid precipitates are used in a self-polishing type antifouling paint, smaller precipitates are quickly hydrolyzed and consumed and larger precipitates are wastefully let out of the coating with the dissolved resin. Therefore, indeed an effective antifouling can be expected with the composition in an early stage, but a long-lasting effect cannot be obtained therewith.
Furthermore, in the method of said Japanese Patent Publication No. 3830/86, an acrylic resin and a metal salt are reacted with each other, each in solution form in water, and loss in solubility of the resin is the only cause of said precipitation. Since the reaction makes steady progress at the surface area of the precipitated resin interacted with said aqueous metal salt solution, the metal ester bondings are always present in a higher concentration at the surface layers of the precipitates. Moreover, the precipitated resins do necessarily have a number of acid groups together with metal ester bondings, because precipitation is occured in an aqueous medium from the decrease in solubility of the resin. They are, therefore, too hygroscopic to use as the resinous filler in a polishing type antifouling paint. For these reasons, a long-lasting antifouling effect cannot be expected with the coating composition added with the disclosed precipitates.
Since the precipitates are not of spherical form, it is hardly possible to maintain the same in a stabilized state of dispersion in a coating composition.
Under such circumstances, the present inventors, having exerted efforts to obtain a novel, spherical form of resin particles which can be added as a resinous filler in a coating composition, thoroughly disintegrated from the interior and exterior of the respective resin particle and which are specifically useful for the preparation of a long-lasting, polishing type antifouling paint, have succeeded in obtaining novel disintegration type crosslinked acrylic resin particles having an average grain diameter of 0.01 to 250 .mu. and containing metal ester bond bearing crosslinks uniformly distributed within the particle bodies. This is achieved by a method wherein a monomer mixture of (A) 5 to 98% by weight of metal ester bond bearing multifunctional polymerizable monomer represented by the formula: ##STR3## (in which R.sub.2 represents hydrogen or methyl group; X is ##STR4## R' is hydrogen, methyl or ethyl group; R is hydrocarbon residue having 1 to 20 carbon atoms; p is 0 or 1 to 5; R.sub.1 is hydrocarbon residue having 1 to 10 carbon atoms; M is a metal whose valency is 2 or more; Y is an organic residue; m and n are positive integers satisfying the conditions: 2.ltoreq.m.ltoreq.q, n=q-m wherein q is equal to the metal valency) and 95 to 2% by weight of mono- or multi-functional polymerizable monomer other than said (A) having at least one .alpha.,.beta.-ethylenically unsaturation bond, in a reaction medium which is unable to dissolve the formed polymer, and applied for patent on it (Japanese Patent Application No. 200,344/86, now laid open as Kokai No. 56510-88/1988).
However, when hydrolyzed, said resin particles can liberate only metal ions, and therefore, if an antifouling paint is designed with these particles alone, the metal source must be necessarily limited to toxic members and the metallic content in the resin must be increased in a moderately higher order. For these reasons, a further development has been desired.
It is, therefore, an object of the invention to provide a novel, disintegration type, spherical form of crosslinked resin particles which can be stably added and dispersed as resinous filler in a resinous varnish for a coating composition, and gradually disintegrated from the exterior and interior of the respective particle in an ionic atmosphere, capable of liberating a bioactive organic acid as well as metal ions. An additional object of the invention is to establish an industrially advantageous method for the preparation of said resin particles. A further object is to provide a novel coating composition containing, as resinous filler, such disintegration type resin particles, which is excellent in film-forming properties, and capable of resulting a coating which is excellent in respect to the self-polishing effects because of the disintegration of said resin particles by hydrolysis in an ionic atmosphere and also in respect to antifouling effects because of the gradual liberation of toxic, bioactive substance besides metal ions. Further objects of the invention shall be clear from the description in the specification and the appended claims.